Gruesome Green Witch by Patricia Coffin

Written in 1969 it features two 11-year-old girls, Puffin and Mole, who meet an assortment of mythical and fictional characters, including the Wizard of Oz, Fafnir (a fearsome dragon of Niebelungenlied), Merlin, King Neptune, a Swedish elf, and the malevolent gruesome green witch. They do their homework in Merlin's concentration cave, where answers are caught as they bounce off walls. (You need to have a license in order to study) . They attend an undersea party presided over by Neptune, with Cinderella, Meg, Jo, Beth and Amy among the revellers, then Puffin incurs the wrath of the villainous, gruesome green witch (who turns her enemies into statues) by seeing her back, and Puffin's brother is captured when the girls bring him into the land so he can profit from the concentration cave. But eventually the witch is conquered by a magic brew which Puffin slips into her tea, and melts down into a pile of green rags.

It is even written in green ink with green illustrations.

Fun and entertaining and a great walk down memory lane as they meet storybook characters during their explorations of the Magic Forest.